Presynaptic plasticity of dopamine release in the cortex and basal ganglia is involved in learning cues that elicit cocaine self-administration and underlie selective attention by filtering non-salient stimuli. The objective of this proposal is to characterize mechanisms that mediate presynaptic plasticity at the level of the dopaminergic synaptic vesicle exocytosis. We recently reported that the fusion pores of small DAergic synaptic vesicles can flicker once or multiple times as fast as 10 kHz, thus regulating the amount of transmitter released from a vesicle. This provides the basis for Aim 1, which asks how flickering of fusion pores is regulated. In chromaffin cells, we discovered that the level of vesicle acidification is not invariant but enhanced by cellular activity, which can increase quantal size. In this proposal, we show evidence for enhanced vesicular acidification that may underlie drug actions (e.g., Ritalin), and a means for behavior to feed back to alter long-term transmission. This provides the basis for Aim 2 explores whether psychostimulants regulate quantal size via rebound of vesicular acidification. Finally, we identified how a variety of presynaptic receptors alter DA transmission at the quantal level (e.g., GDNF, D2, nicotinic, and mGluR activation) and why these factors are important for DA as social synapses. Most of the work has been on acute effects of neurotransmitters. New evidence indicates long- lasting effects of neurotransmitters on DA transmission, particularly on the density/formation of axonal presynaptic varicosities. Accordingly, Aim 3 will examine how neurotransmitters regulate DAergic presynaptic varicosity formation.